The invention relates to a thermal-acoustic protection sheet assembly for an exhaust duct of a rotary machine or boiler associated with the machine.
Rotary machines, such as industrial gas turbines, and boilers discharge exhaust gases that require exhaust ducts. The exhaust gases typically leave the rotary machines at high velocities and high temperatures such as in excess of 600° C. (degrees Celsius). Exhaust ducts, such as diffusers, include passages that direct the exhaust gases from the rotary machines or boilers to heat recovery steam generators (HRSG) or other systems that receive the exhaust gases.
Exhaust ducts tend to be relatively large structures that provide a gas passage for large volumes of exhaust gases. A typical exhaust duct may have a cross-sectional dimension of 100 to 900 square feet (9 to 100 square meters). The cross-sectional area of the exhaust duct may increase as the exhaust gas flows from the rotary machine to the HRSG. The exhaust duct may have a generally square, circular or conical cross section. The exhaust duct may be connected to the discharge end of the rotary machine or boiler by an expansion joint.
The walls of an exhaust duct are typically formed by insulated sheet assemblies. These assemblies are arranged side-by-side and supported by a framework of ribs to form the wall of a passage for the exhaust gases. Each sheet assembly includes an internal stainless steel sheet, an external steel sheet and an intermediate layer (or layers) of insulation between the internal and external sheets. Insulation in the walls suppresses the transmission of heat and acoustic noise from the hot gases to the environment surrounding the exhaust duct.
The insulation in the intermediate layer is typically formed of mineral fibers, glass fibers, bio-soluble fibers or rock wool. The insulation is arranged in layers or may be packed as a cushioning material between the interior and external sheets. The materials and thicknesses of the intermediate layer(s) of the exhaust duct is typically selected to attain a certain low heat transfer coefficient through the intermediate layer. The insulation is generally configured to prevent the material from disintegrating over time and facilitate the handling of the insulation during installation of the insulation into the sheet assemblies.
The sheet assembly includes support plates also referred to as separators that extend through the intermediate layer to support the internal and external sheets and provide spacing for the intermediate layer.
The support plates are typically perpendicular to the internal and external sheets. An edge of each support plate is fixed or welded to an internal surface of the external sheet. An opposite edge of each support plate is attached to a bolt that extends through the internal sheet. A nut and washer are fastened to the bolt to secure the internal sheet to the support plate. The assembly of nuts, washers and support plates form a framework that holds together the internal sheet, the intermediate layer and the external sheet.
The bolts and support plates tend to conduct heat through the sheet assembly. Heat is conducted because the bolts and support plates form a metal path through the intermediate layer. The bolts are fasten to the hot internal sheet and may be directly exposed to the hot exhaust gases if they extend through the internal sheet. The bolts conduct heat to the support plates. If an edge of the support plate is in contact with the internal sheet, heat from the internal sheet may be conducted directly to the support plate. The metal material of the bolts and support plates conducts heat to a much greater extent than does the insulation of the intermediate layer. The metal paths of the bolts and support plates form thermal bridges through the insulation of the intermediate layer of the sheet assembly.